Use timer instead of autocmd to defer startup processing

If the user has removed the "gitgutter CursorHold" autocommand, which
is the documented way to turn off automatic refreshing, calling the
autocommand at start up will fail.  Use a timer instead.

This implementation also changes the processing delay to 1ms instead of
'updatetime'.

See #840.

plugin/gitgutter.vim

@@ -245,8 +245,8 @@ function! s:on_bufenter()
   " been any changes to the buffer since the first round, the second round
   " will be cheap.
   if has('vim_starting') && !$VIM_GITGUTTER_TEST
-    if exists('*timer_start')
-      call timer_start(&updatetime, 'GitGutterCursorHold')
+    if exists('*timer_start') && has('lambda')
+      call s:next_tick("call gitgutter#process_buffer(+".bufnr('').", 0)")
     endif
     return
   endif
@@ -259,10 +259,6 @@ function! s:on_bufenter()
   endif
 endfunction
 
-function! GitGutterCursorHold(timer)
-  execute 'doautocmd' s:nomodeline 'gitgutter CursorHold'
-endfunction
-
 function! s:next_tick(cmd)
   call timer_start(1, {-> execute(a:cmd)})
 endfunction